1. Field of the Invention
This invention relates to articulated fluid transferring apparatus and, more particularly, to a system for transferring fluid between two marine tankers.
2. Description of the Prior Art
The production of oil and gas from offshore wells has developed into a major endeavor of the petroleum industry, and this growth has led to the development of means for transporting petroleum products from offshore wells to shore-based refineries or storage facilities which may be several hundreds or thousands of miles from the loading area. Many of the wells are in deepwater locations where the use of marine tankers of very large capacity constitutes the most practical and efficient method of transporting the petroleum products. Unfortunately, some of the storage and refinery facilities are located near harbors having small or shallow unloading areas where large marine tankers cannot be safely maneuvered.
One solution to this problem is to position the large tankers in deepwater some distance from the smaller or shallow harbors, and then transfer the petroleum products to tankers of smaller draft and/or size that permits their safe use in such limited harbors. However, during the fluid transfer operation the large and small tankers move relative to each other due to winds, tides, and currents, and also in response to a change in the amount of fluid in each of the tankers, and draft of each tanker changes as it is being loaded or unloaded, thus requiring the use of an articulated or other flexible fluid transfer conduit between the two tankers. When flexible hoses are used a tender is normally required to assist in positioning the hoses for connection to a tanker manifold. Not only is the use of tenders costly, time consuming, and otherwise undesirable, but movement of the tankers may cause the flexible hose to rupture. Also, such hoses are bulky, heavy, and otherwise hard to handle, and require a relatively large crew of workers to make up their connections to the tankers.
Another solution to the ship-to-ship fluid transfer problem is to mount an articulated loading arm on the larger tanker and connect this loading arm to either a manifold or a second articulated loading arm located on the smaller tanker. Articulated fluid loading arms generally comprise an inboard limb pivotally supported on a vertical riser pipe to facilitate movement of the arm about horizontal and vertical axes, an outboard limb connected by a pipe swivel joint to the inboard limb for pivotal movement relative thereto about another horizontal axis, and a coupler or other device at the outer end of the outboard limb for releasably connecting the arm to a tanker manifold or a second articulated arm. Because of their large size, the inboard and outboard limbs are individually maneuvered and controlled by hydraulic systems, and the operation of these control systems must be coordinated in order to properly position the outer end of the arm. When a pair of loading arms are each mounted on a separate tanker, and the tankers are in motion due to the action of the sea, etc., the task of aligning and interconnecting the outer ends of the two arms by manual operation of the hydraulic controls is very difficult.
Accordingly, what is needed is an apparatus for sensing the position of each of the outboard ends of the two loading arms relative to a reference position on one of the tankers, means for moving the outboard ends into alignment, and means for coupling the outboard ends together in a fluid-tight connection. This requires sensing the horizontal and vertical angle of the outboard end of each loading arm relative to the reference position, and sensing the distance between the reference position and each of the outboard ends. The position of each outboard end relative to the reference position can be used to calculate the position of one outboard end relative to the other outboard end, and the hydraulic controls operated to bring the two outboard ends together.
If the outboard end of either of the loading arms is extended beyond a safe distance from the mounting base of the arm, or if a loading arm is moved vertically or horizontally outside a three-dimensional space known as the arm's "operating envelope", the weight of oil or other fluid in the arm during use could cause damage to the arm or to a manifold to which the arm is connected. To keep the stress of the arm on the manifold within acceptable limits, the extension of the arm and the slew angle must be limited. It is therefore important that a system be provided for monitoring the actual position of the outboard end of the arm and for sounding an alarm whenever the end of the arm extends outside the specified envelope, or whenever the stress on the arm exceeds a predetermined value.
One solution to the problem of monitoring the actual position of the end of the loading arm is to employ an electronic computer which can use the elevation angle of the inboard and outboard limbs, the arm's slew angle, and the lengths of the inboard and outboard limbs to calculate the actual spatial position of the end of the arm and to compare this position with a set of safe spatial boundaries. The computer develops a warning signal when the end of the loading arm extends outside any of the safe boundaries. One such system is described in Haley U.S. Pat. No. 4,205,308, issued on May 27, 1980.